EP-A-1 211 273 discloses 1K [one-component] coating compositions which in addition to hydroxyl-containing binders comprise as crosslinker the reaction product of aliphatic and/or cycloaliphatic polyisocyanates with at least one secondary 3-aminopropyltrialkoxysilane and at least one blocking agent. Silane compounds used are preferably N-alkyl-3-aminopropyltrialkoxysilanes, more particularly N-butyl-3-aminopropyltrimethoxysilane. These coating compositions are notable for a crystallization tendency on the part of the blocked isocyanates which is less than that of conventional 1K coating compositions based on nonsilanized blocked isocyanates, and also an improved scratch resistance on the part of the resultant coatings. A particular disadvantage, however, is the inadequate storage stability of the coating compositions.
Also known, from WO08/110,229 and WO08/110,230, are 1K and 2K [two-component] coating compositions which in addition to hydroxyl-containing binders and amino resins and/or isocyanates comprise as crosslinker the reaction product of isocyanates with surface-active compounds and aminoalkyltrialkoxysilanes. The reaction products containing silane groups accumulate at the surface of the resultant coatings, where they lead to an increased crosslinking density as compared with the bulk. As a consequence of this, the coatings obtained are notable for enhanced scratch resistance and micropenetration hardness in conjunction with very good weathering stability and crack resistance. A parameter in need of improvement is the storage stability of the 1K coating compositions.
Additionally known, from WO05/092934, are 1K coating compositions, more particularly for automotive refinish, which comprise hydroxyl-containing poly(meth)acrylates blocked with orthoesters, as binders, and di- and/or polyisocyanates having free isocyanate groups, as crosslinkers. The coating compositions are notable for a reduced organic solvent content. Parameters in need of improvement are the scratch resistance of the resultant coatings and also the storage stability of the coating compositions.
Disclosed, furthermore, by EP-B-1 225 172 are coating compositions which comprise polyorthoesters (A), crosslinking agents (B) having groups that are reactive toward hydroxyl groups, such as blocked or nonblocked isocyanates, amino resins, epoxy resins, anhydrides or components containing alkoxysilane groups, and optionally hydroxyl-containing compounds (C). The polyorthoesters (A) are reaction products of (a) at least one monomeric α-glycol and/or β-glycol, (b) hydroxyl-containing oligomeric and/or polymeric compounds other than the glycols (a), such as hydroxyl-containing poly(meth)acrylates, with (c) orthoesters of C1 to C4 carboxylic acids, such as, more particularly, orthoformates and orthoacetates. Through the use of α-glycol and/or β-glycol in the preparation of the polyorthoesters, the aim is to avoid gelling of the polyorthoesters and to lower the viscosity of the resultant polyorthoesters. Information as to how the storage stability of the resulting coating compositions can be improved while nevertheless ensuring effective curing of the coating compositions is absent here, as is information on enhancing the scratch resistance of the resulting coating. Furthermore, the preparation of the polyorthoesters used therein is decidedly costly and inconvenient.
U.S. Pat. No. 6,297,329, finally, discloses coating compositions which comprise hydroxyl-containing monomeric or polymeric compounds blocked with bicyclic orthoesters or with spiro-orthoesters, optionally nonblocked hydroxyl-containing compounds, and crosslinking agents having groups that are reactive toward hydroxyl groups. As a result of the blocking with the specific orthoesters, the viscosity of the binders is lowered in comparison to the viscosity of the nonblocked binders, the aim of this being to allow provision of coating compositions having a relatively low solvent content. In addition, the sagging behavior of the coatings is improved, because the viscosity of the coating compositions after application is increased by virtue of the fact that, as a result of contact with atmospheric humidity, the orthoesters are eliminated and hence the hydroxyl-containing binders, which have a higher viscosity, are released again.
Examples of suitable groups in the crosslinking agents that are reactive toward hydroxyl groups are specified in U.S. Pat. No. 6,297,329 as being blocked or nonblocked isocyanate groups, acetal groups, epoxide groups, carboxyl groups, anhydride groups and/or alkoxysilane groups. Alternatively as crosslinking agents it is also possible to use amino resins. Information on how, through the selection of the crosslinking agents, it is possible to direct the properties of the resulting coatings, such as the scratch resistance, weathering stability, and acid resistance, for example, is absent, however, from the text. Here again, moreover, the storage stability of the coating compositions is a parameter in need of improvement.
The problem addressed by the present invention was that of providing coating compositions, more particularly for the clearcoat in automotive OEM finishing and in the finishing of parts for installation in or on automobiles, that have a very good storage stability, meaning that the viscosity on storage at temperatures of 40° C. must not increase by more than 50% within 28 days. At the same time, the coating compositions ought also to ensure very good curing under the curing conditions that are typical in the automobile finishing segment, more particularly at temperatures of 100 to 200° C. for a time of 10 minutes up to 1 hour.
Furthermore, the coating compositions ought to result in high-gloss coatings which are highly scratch resistant and in particular exhibit a high gloss retention after scratch exposure. Moreover, the resultant coatings and coating systems, especially the clearcoat systems, ought to be suitable for use even for the technologically and aesthetically particularly demanding field of automotive OEM finishing. Hence the coatings ought also to have a good acid resistance and low yellowing on baking. Furthermore, the new coating compositions are to be preparable easily and very reproducibly, and are not to cause any environmental problems during application of the coating material.